i) Field of the Invention
This invention relates to a process for producing chlorine by reacting hydrogen chloride and oxygen in the presence of a chromium oxide catalyst and also to a reactor useful in the practice of the process. More specifically, this invention is concerned with the material of a reactor which is employed for the oxidation of hydrogen chloride with oxygen.
ii) Description of the Related Art
Chlorine is produced on a large scale by the electrolysis of brine in order to meet the ever-increasing demand therefor. Caustic soda is also produced along with chlorine in the electrolysis of brine.
Since the demand for caustic soda is lower compared with that for chlorine, the production of chlorine by the electrolysis of brine is confronting a difficult situation that it can hardly cope with this imbalance in demand between these compounds.
On the other hand, a great deal of hydrogen chloride is byproduced upon chlorination or phosgenation of organic compounds. Because such byproduction of hydrogen chloride is far greater than the market demand for hydrochloric acid, a substantial portion of hydrogen chloride is disposed wastefully without its effective utilization. The cost for its disposal has also come up to big figures.
If chlorine can be recovered efficiently from hydrogen chloride which is thrown away in a large volume as described above, the demand for chlorine can be met by using such chlorine along with chlorine produced by the electrolysis of brine in view of the demand for caustic soda.
The reaction in which hydrogen chloride is oxidized to produce chlorine has been known for many years. It has also been well-known to use chromium oxide as a catalyst in such a reaction.
It has recently been found that a chromium oxide catalyst obtained especially by calcining chromium hydroxide has good activity even at relatively low temperatures, whereby the possibility of industrial practice of a process for the production of chlorine by the oxidation of hydrogen chloride has been enhanced further. However, some unsolved problems still remain regarding the maintenance of its catalytic activity.
The present inventors have carried out a variety of investigation with a view toward providing a solution to the above problems. In the course of the investigation, a phenomenon was observed that the catalytic activity decreased gradually in a long-hour reaction even when a chromium oxide catalyst was employed. It has then been found that this phenomenon is attributable to the poisoning of the catalyst by iron. Namely, it has been found that the chromium oxide catalyst is prone to iron poisoning, is poisoned even by iron contained in a small amount in the reactor materials to which the catalyst is brought into contact, and is hence difficult to maintain its high activity for a long period of time. It has also been found that the activity reduction of the catalyst takes place in a long period of time even when a metallic material containing iron only in a very small amount, for example, a high nickel alloy steel is used.